Biocontrol of sclerotinia stem rot ( Sclerotinia sclerotiorum ) of soybean using novel Bacillus subtilis strain SB24 under control conditions

Sclerotinia stem rot (SSR), caused by Sclerotinia sclerotiorum , is a major disease of soybean in Canada. Laboratory and greenhouse experiments were conducted to evaluate potential effectiveness of cell suspensions, cell‐free culture filtrates and broth cultures of Bacillus subtilis strain SB24 for suppression of SSR. The SB24 cell suspensions and cell‐free culture filtrates significantly reduced mycelial growth of S. sclerotiorum by 50 to 75% and suppressed sclerotial formation by > 90%. The severity on soybean was negatively correlated ( r < −0·84, P < 0·01) to the concentrations of cell suspension, cell‐free culture filtrate and broth culture applied. The cell suspension and broth culture preparations significantly ( P < 0·01) reduced SSR severity by 45 to 90% at concentrations ranging from 5 × 10 6 to 10 9 CFU mL −1 . The most effective concentration was 5 × 10 8 CFU mL −1 for all three preparations, reducing the severity by 60 to 90%. The B. subtilis SB24 was most effective in reducing disease severity when applied ≤ 24 h before plant inoculation with S. sclerotiorum and a significant effectiveness was observed up to 15 days after plant inoculation. The population density of B. subtilis on soybean leaves decreased by 1·5 to 2·5 log units over 15 days under field conditions, and by 0·8 log units over 5 weeks under control conditions. The decrease in population density was significantly correlated with rainfall in the field ( r < −0·93, P < 0·01), suggesting that the biocontrol bacteria may be washed away by rain.